This invention relates to the field of throttle position sensors and more particularly to a potentiometric device for determining the position of the butterfly valve in the carburetor of an internal combustion engine.
Throttle position sensors are known in the art for sensing the position of the butterfly valve of the carburetor and thus the position of the throttle. This information is typically used to control a fuel injection system or as one input of an electronic system for monitoring engine performance. Such a system can, for example, aid in improving gas mileage. The term "butterfly valve" is used broadly throughout this specification to refer to any fuel control apparatus.
Prior art potentiometric throttle position sensors are generally characterized by a single assembly in which stationary resistive element and movable wiper contacts connected to a rotor are mounted within a two piece casing. The rotor is mounted for rotation in the casing by using two bearings mounted in the casing at opposite ends of the rotor. One end of the rotor sticks out of the casing slightly and is provided with a actuator arm that makes contact with a similar amr connected directly to the butterfly valve. The connection is such that when the butterfly valve is opened the actuator arm is rotated, which in turn acts to rotate the rotor.
In order to prevent the possibility that failure of the sensing element might possibly lock the butterfly valve in an open throttle position, safety regulations require that there be no direct connection between the potentiometer and the butterfly valve. Thus the movement of the arm connected to the butterfly valve only acts to rotate the rotor when the throttle and butterfly valve are being opened and the rotor is not directly rotated by the closing movement of the butterfly valve. However, it is desirable to follow the rotation of the butterfly valve when it closes so that an accurate indication of the position of the throttle is given. This is provided by a torsion spring inside the throttle position sensor casing concentric with the rotor. The torsion spring is tensioned such that as the butterfly valve closes the rotor will be turned to follow this movement.
These prior art potentiometric sensors have certain disadvantages. For example, the fact that the rotor is mounted with dual bearings in the casing, one at each end, makes alignment of the rotor very difficult. Proper alignment requires both that the rotor be concentric with the bearings and that the rotor be perpendicular to the plane of the resistive element. Where one or both of these requirements are not met, the potentiometer can exhibit "geometry" errors, i.e., the amount of movement of the wiper contacts over the resistive element for a given number of degrees (for example 10.degree.) may vary depending upon the relative position of the wiper contacts upon the resistive element. In some applications this accuracy can be crucial. Furthermore, the structural configuration of the prior art devices are such that manufacturing tolerances can easily result in misalignment between an external "pilot" diametric integral with the case of the position sensor and the rotor's axis of rotation. As a result, it is either not possible to properly attach the sensor to the carburetor or only be expending additional time during the assembly of the units.
Additionally, because the rotor uses two bearings located one at each end of the rotor, the connector terminals must be located off to the side of the rotor. In addition, position sensors made as single assemblies have the disadvantage that, if after testing it is determined that the potentiometer is defective, the entire assembly must be disposed of or re-worked.